CORTICAL MECHANISMS UNDERLYING TACTILE DISCRIMINATION IN THE MONKEY I. ROLE OF PRIMARY SOMATOSENSORY CORTEX IN PASSIVE TEXTURE DISCRIMINATION. F. Tremblay, S. A. Ageranioti-Belanger and C.E. Chapman. Centre de recherche en sciences neurologiques, Departement de physiologie and ecole de r[acute]eadaptation, Faculte de m[acute]edecine, Universite de Montreal, Montreal, Quebec & ecole des sciences de la readaptation, Universit[acute]e d'Ottawa, CANADA.
APStracts 3:0160N, 1996.
1. The discharge patterns of 359 single neurones in the hand representation of primary somatosensory cortex (SI) of two monkeys ( macaca mulatta ) were recorded during the performance of a passive texture discrimination task with the contralateral hand (104 in area 3b, 149 in area 1, and 106 in area 2). Three nyloprint surfaces were mounted on a drum that was rotated under the digit tips. One surface was entirely smooth , while the other two were smooth over the first half and rough over the second half, smooth/rough (raised dots, 1 mm high, 1 mm in diameter, in a rectangular array; spatial period of 3 mm across the rows and columns for most recordings; 9 mm between rows for selected recordings). The monkeys were trained to distinguish between the smooth and smooth/rough surfaces. After the surface presentation, the monkey indicated the texture of the second half of the surface by, respectively, pushing or pulling on a lever with the other arm. For most recordings an average tangential speed of 49 mm/s was tested. For selected recordings, motor speed was incremented (63, 75 or 89 mm/s). 2. Two hundred and eighty three neurones had a cutaneous receptive field (RF) on the hand (96 in area 3b, 120 in area 1, and 67 in area 2). Thirty-five neurones had a deep RF (4 in area 3b, 15 in area 1, and 16 in area 2). Seven neurones had mixed cutaneous and deep RFs (4 in area 1, 3 in area 2). Thirty-four neurones had no identifiable RF (4 in area 3b, 10 in area 1, and 20 in area 2). 3. The discharge of 185/359 neurones was significantly modulated during the presentation of one or both surfaces, as compared to the discharge at rest. Cells with a cutaneous RF that included part or all of the distal phalangeal pads of the digits used in the task (usually digits III and IV) were more likely to be modulated during surface presentation (132/179, 74%) than those with a cutaneous RF not in contact with the surfaces (24/104, 23%). The remaining neurones (mixed, deep or no RF) were also infrequently modulated (29/76, 38%). 4. One hundred and eighteen cells (out of the 185 modulated units) were classified as texture- related because there was a significant difference in the discharge rate evoked by the smooth/rough and smooth surfaces. Cells with a cutaneous RF that included the digital pads in contact with the surfaces were frequently texture-related (100/132, 76%). Texture-sensitivity was less frequently observed in the remaining modulated neurones (18/53, 34%: cutaneous RF not in contact with the surfaces, deep RF, mixed cutaneous and deep RF, no identifiable RF) 5. Texture-related neurones were found in areas 3b, 1 and 2. Two patterns of texture-related responses were observed in the 100 cutaneous units with a RF in contact with the surfaces. Thirty one units were classified as showing a phasic response at the time the digits encountered the leading edge of the rough half of the surface. Fifty eight cells were classified as phasic-tonic (or sometimes tonic at the slowest motor speeds) since the response lasted for the duration of the presentation of the rough portion of the surface. The remaining 11 neurones could not be readily classified into one or the other category and, indeed, generally showed clear texture-related responses only at higher motor speeds (>49 mm/s, 9/11). 6. Speed sensitivity was systematically evaluated in 41/100 texture-related units with a cutaneous RF in contact with the surfaces. The discharge of 66% of the units (27/41) varied significantly with the speed of surface presentation, with discharge increasing at higher speeds. Speed-sensitivity was found in all three cytoarchitectonic areas (6/6 in area 3b, 11/22 in area 1, and 10/13 in area 2). 7. Contact force was also systematically monitored in these experiments (69/100 texture-related cells with a cutaneous RF in contact with the surfaces). Linear regression analyses indicated that 22% (15/69) of the texture-related units were sensitive to contact force (13 positive, 2 negative). For some of these units, however, discharge also covaried with speed (8/10). Since contact force was not always clearly independent of speed, the relative importance of contact force in determining the discharge pattern of neurones involved in the appreciation of surface texture remains unclear. 8. The present results suggest that texture is a distributed function across areas 3b, 1 and 2. Phasic-tonic units likely provide information about the characteristics of the scanned textures, while the phasic responses may represent a form of feature-selectivity in SI cortex. Neurones that signal differences in the texture and yet are invariant for speed may provide the neuronal basis for the perceptual constancy of texture across a range of different speeds.

Received 19 December 1996; accepted in final form 24 June 1996.
APS Manuscript Number J852-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 21 August 1996